Sputter coating involves the process of ionically bombarding the surface of a target of sputtering material to eject from it particles of atomic dimension which redeposit onto the surface of a substrate as a thin film. The process is carried out in a vacuum chamber utilizing a neutral gas such as argon for the source of ions. Ionization is achieved by biasing the target with a negative potential to cause electrons to be released from the target and to move toward an anode. In the course of this motion, the electrons collide with and ionize atoms of the gas above the target surface. The positive ions thus formed are attracted toward the target surface by its negative charge and, upon impact, transfer momentum to the target surface and eject atoms or small particles of coating material from the target. The ejected atoms move from their points of emission on the target surface and strike and adhere to the portions of the substrate surfaces in their paths.
Magnetron sputtering is an enhanced sputtering method in which a magnetic field is formed over the target surface. The field deflects electrons emitted from the target so that they move in confined paths and are thus trapped in confined spaces above the target surface. The confinement of electrons greatly increases their density and the likelihood that collisions of electrons with atoms of the gas in the space near the target surface will occur, thus increasing the useful production of ions. The concentration of ions so produced is manifested in the form of a glowing plasma in the confined space above the target surface and results in a higher rate of ion bombardment of the region of the target surface adjacent the plasma This causes an increased emission rate of sputtering material and thus a more rapid erosion of the target surface adjacent the plasma.
A main problem in the sputtering technology of the prior art has been in achieving uniformity of the coating applied to the substrate surface. The substrates are often wafers which are to be coated with conductive material to form electronic circuits. Prior to the performance of the sputter coating process, wafers are frequently processed by other coating or etching processes to prepare them for the deposition of multiple circuit layers. These processes result in the formation of linear grooves in the surface layers or in holes (called "vias") therethrough, the vertical sides of which are perpendicular to the planar surface of the wafer. In the sputter coating process, these sides or "steps" must also be coated to provide for electrical conduction between various conductive circuit layers joined by the stepped surfaces. As a result, the problem of uniformly coating the wafer is complicated by the need to uniformly coat mutually perpendicular and differently facing portions of the wafer surfaces in the sputtering process.
The sputter coating devices of the prior art have encountered different types of problems in providing the desired distribution of the coating applied to the substrates Some proposed solutions to certain problems have aggravated other problems.
Planar targets have been employed with plasmas confined by magnetic fields over the target area. See U.S. Pat. Nos. 3,878,085 and 4,166,018. Attempts to smooth the erosion of the target have been frequently made in the prior art. Moving magnetic fields have been employed for this purpose, and this has been attempted by both mechanically moving magnet elements and by electrically moving magnetic fields by changing magnet currents. See U.S. Pat. Nos. 3,956,093 and 4,401,539, and Japanese Publication No. 58-71569.
Some prior art devices have made efforts to spacially adjust the flux of a sputtered coating material to compensate to some degree for non-uniformity of coverage of the substrate which would otherwise occur. Such a technique is shown in U.S. Pat. No. 4,747,926, for example. This effort has led in the prior art to the provision of separate isolated targets with separately regulated power supplies. Such targets, such as those shown in U.S. Pat. Nos. 4,606,806 and 4,595,482, may utilize a planar target surrounded by an annular frustoconical target electrically isolated from the planar target. When using separately isolated targets, each target may have a magnetic field over its surface and a plasma which is separate and distinct from that affecting the other target surface. A separate power source may be used to independently energize each target part, as shown in U.S. Pat. No. 4,595,482.
Plural targets, however, require multiple power components and duplicative circuitry and controls for each of the target cathode power supplies and for each of the magnetic field generating current power supplies. From a mechanical point of view, these plural targets also require separate seals to maintain the vacuum in the chamber, require separate installation and alignment procedures, require separate manufacturing steps, and require separate means to insure that the targets are properly cooled. All of these requirements result in generally greater cost, increased manufacturing and maintenance problems, and greater complexity in the preparation and operation of the machines
One piece targets of the prior art have not been amenable to regulation of the sputtering intensity from different regions on the sputtering surface. Lack of an ability to effectively control emissions from separate target regions has been a disadvantage of one piece targets of the prior art, thus motivating the development of multiple target assemblies with their inherent disadvantages.
Prior art developments directed at single targets or at individual target components of target assemblies have concentrated on preventing non-uniform erosion, as discussed in U.S. Pat. No. 4,401,539. Target surfaces often exhibit undesirable erosion patterns which alter the target surface geometry causing a departure from the initial emission pattern of the target. Consequently, the deposition distribution on the substrate also changes as the target erodes. Furthermore, the variation of the emission strength and across the target surface and the resulting variation in deposition uniformity across the substrate continue to change with time. Thus, irregular erosion has been regarded in the prior art, for example, in U.S. Pat. No. 4,100,055, as a phenomenon to be prevented. Reconfiguration of magnet poles or moving of pole pieces has been employed to smooth target erosion, for example, as shown in U.S. Pat. No. 4,622,121. In the prior art, non-uniform erosion of single targets has been regarded as resulting in an inefficient use of target material due to the non-uniform consumption of the target across its surface.
Erosion also results in a change, generally a decrease, in the sputtering rate as the target erodes. With non-uniform target erosion this decrease occurs non-uniformly across the sputtering surface and results in a time varying change in the distribution of sputtering power on the target surface and a resulting change in the total amount and the distribution of coating material onto the substrate. With separate electrically isolated targets, these effects can be measured and compensation made through control of electrical parameters. But with single targets or target components, such effects occur in a way which heretofore could not be measured or controlled during the course of the sputtering process. Visual inspection of the target and adjustment of electrical parameters of the target based on experience in observing the target erosion was the only course which the prior art employed. For example, U.S. Pat. No. 4,166,783 relates to one attempt at such control. Accordingly, the goal of intentionally causing non-uniform emission rates from target surfaces was primarily restricted to the use of multi-part targets, and was inconsistent with the goal of controlling erosion patterns on a one piece target. Non-uniform emission patterns necessarily produce non-uniform target erosion.
The need to uniformly coat substrates having steps and vias with sides perpendicular to the substrate wafer plane has been inadequately dealt with in the prior art. Non-uniform target emission rates and target erosion control remain problems in the prior art, particularly for one piece targets.
Accordingly, there is a need for providing, maintaining and controlling magnetron sputtering targets for uniformly coating substrate wafers with steps. Furthermore, there is a more specific need to utilize heretofore incompatible features to solve the problems of the prior art discussed above, particularly in a one-piece sputtering target.